Molded articles, and particularly blow molded structures such as bottles are commonly formed from polymers of 1-olefins such as polyethylene. It is important to the commercial utilization of a given polymer system that the converted product such as a bottle exhibit an optimized balance of properties, including for example, acceptable stress crack resistance and flexural stiffness. In addition, and in a contributing sense, it is necessary that the polymer exhibit suitable processability i.e. satisfactory rheological behavior under flow and deformation during fabrication. Although the viscoelastic behavior of polymer melts has been the subject of considerable study, it has not proven possible to translate performance during fabrication to end use articles in such manner as to selectively determine polymerization and particularly catalyst requirements. Moreover, as in any case catalyst performance must also be measured in terms of efficiency or productivity and stability over a sensible life.
In the following description, supported catalytic systems for the production of polymers ideally suited to the fabrication of superior blow molded articles have been identified. In accordance with this invention, there are disclosed supported catalyst compositions incorporating preformed materials comprising the reaction product of organophosphorus compounds such as the organophosphates and organophosphites with chromium trioxide, wherein the organic moiety is a hydrocarbon radical, e.g., alkyl, aralkyl, aryl, cycloalkyl, or the like or combinations thereof. Typical supports constitute an inorganic material of high surface area, especially a high pore volume (&gt;1.96 cc/g) silica xerogel. Catalytically promoted polymerizations of 1-olefins proceed efficiently to products of specially adapted utility for molding, and particularly the production of blow molded articles. The polymerization activity of the catalyst compositions is promoted by heating in a dry, oxygen containing atmosphere. The catalysts are utilized alone or in conjunction with other catalytic ingredients such as organometallic and/or organo non-metallic reducing agents.
In U.S. Pat. No. 3,474,080 issued Oct. 21, 1969 in the name of Louis J. Rekers and assigned to National Distillers and Chemical Corporation there were described compounds useful in the catalysis of 1-olefins which were prepared by the reaction of organophosphates and chromium trioxide.
Through further research it has now been found that such compounds lend themselves to the production of supported catalyst systems which are useful for the polymerization of 1-olefins, and the polymerization activity of which is strongly promoted by heating of the catalyst in a dry oxygen containing atmosphere. By an extension of this research it was also found that the resulting activated catalysts, used alone or in conjunction with certain reducing agents, produce polymers of 1-olefins and copolymers or interpolymers thereof having a very favorable blend of properties. The use of the organometallic and/or organic non-metallic reducing agents with the air/heat treated catalyst gives latitude in obtaining more variety in polymer property balance in addition to promoting catalytic activity.
In another U.S. Pat. No. 3,493,554 issued Feb. 3, 1970 in the name of Louis J. Rekers and assigned to National Distillers & Chemical Corp., there was disclosed the polymerization of 1-olefins in the presence of a reducing agent and a bis (diorgano) chromate compound as a catalyst.
Other researchers have also investigated certain other chromium compounds and phosphorous compounds and their use in olefin polymerization. For example, in their U.S. Pat. No. 2,825,721, issued on Mar. 26, 1956, Hogan et al described a polymerization process for olefins using as a catalyst chromium oxide and at least one material selected from the group consisting of silica, alumina, zirconia, and thoria, at least part of the chromium being in the hexavalent state at the initial contact of the hydrocarbon with the catalyst. In related U.S. Pat. No. 2,951,816 issued Sept. 6, 1960 Hogan et al described a method for preparing such catalysts by depositing chromium oxide on a support of the just recited group and heating at an elevated temperature under anhydrous conditions to impart increased catalytic activity.
In U.S. Pat. No. 2,945,015 issued July 12, 1960 in the name of Clyde V. Detter a process is described for polymerizing 1-olefins using a chromium oxide-phosphorous oxide supported catalyst, at least a portion of the chromium in the catalyst being in the hexavalent state. Supported chromium catalysts of limited productivity are disclosed in U.S. Pat. No. 3,349,067 as esters of chromyl chloride with e.g. tricresyl orthophosphate.
Other researchers have reported the use as catalysts for olefin polymerization of various silylchromate and polyalicyclic chromate esters. See, inter alia, U.S. Pat. Nos. 3,324,095 and 3,324,101, both issued on June 6, 1967; 3,642,749 issued Feb. 15, 1972; and 3,704,287, issued Nov. 28, 1972, all assigned to Union Carbide Corporation. The last of this group discloses placing the phosphorus chromate esters of U.S. Pat. No. 3,474,080 on a support and then reducing the catalyst prior to contact with the olefin by heating at elevated temperatures in the presence of an aluminum, magnesium or gallium organometallic compound.
The preparation and use of improved high pore volume silica xerogel materials suitable as catalyst supports is described in Belgium Pat. No. 741,437 and U.S. Pat. Nos. 3,652,214; 3,652,215 and 3,652,216, assigned to National Petro Chemicals Company, Inc.
Although such chromium catalysts, support media, and combined systems have been available for use in this art, and may in selected manner be employed to prepare polymers suitable for conversion into molded articles such as bottles, none are found to provide systems operable to afford the desirable features appurtenant the teaching of this invention.